FIG. 1 is a cross-sectional view showing a conventional laminate wood flooring in which a lower HPL or a glass fiber layer is laminated instead of the lower HPL. As shown in FIG. 1, the conventional laminate wood flooring has a structure in which an upper HPL 30 is laminated to balance between upper and lower portions or a glass fiber layer 24 is laminated instead of the upper HPL 30, and an upper HPL 20′ and a waterproof plywood layer 10′ are adhered to each other by an adhesive layer 40.
The upper HPL 20′ includes 1-3 sheets of core layers (23 and 25) which can be laminated to each other depending on the desired thickness. 30 Alternatively, for improved adhesion to the floor and economical efficiency, the glass fiber glass 24 can be added instead of the lower HPL 30, so that the upper HPL 20′ includes the second core layer 25, the glass fiber layer 24, the first core layer 23, a decorative paper layer 22 and a surface-protective paper layer 21 laminated in this order from the bottom. The respective layers constituting the 35 conventional laminate wood flooring are briefly explained below with reference to FIG. 1.
The waterproof plywood layer 10′ is produced by layering 5 to 7 veneers together using a phenol or melamine resin adhesive in such a manner that the grain directions of the veneers are at right angles to each other, and pressing the laminate in a press.
The core layers constituting the upper HPL 20′ are produced by impregnating in a kraft paper a phenol resin and a melamine resin. Two or more sheets of the kraft paper can be used depending on the desired thickness.
The core layer 23 is produced by impregnating a kraft paper in a phenol resin, the decorative paper layer 22 is produced by impregnating a decorative paper having a printed pattern in a melamine resin, and the surface-protective paper layer 21 is produced by impregnating an overlay paper in a melamine resin. At this time, the overlay paper is prepared by uniformly distributing wear-resistant silica or alumina particles in a cellulose paper.
Since the conventional laminate wood flooring in which the HPL 20′ is laminated on the waterproof plywood 10′ uses the waterproof plywood layer 10′ instead of a medium-density fiberboard (MDF), it exhibits excellent dimensional stability against heat and moisture. In addition, since the surface-protective paper layer 21 and the decorative paper 22 impregnated in a melamine resin, the conventional laminate wood flooring exhibits excellent surface physical properties.
In addition to the physical properties, the temperature stability of the conventional laminate wood flooring is measured by heating the conventional laminate wood flooring at 80° C. for 24 hours. The degree of shrinkage is measured and expressed in percentage (dimensional stability), and the occurrence of the curls is measured and expressed in length (curling property). The dimensional stability of the conventional laminate wood flooring is shown to be −0.1˜0.2% and curling property is shown to be 0.5˜5.0 mm. Further, after installation of the conventional laminate wood flooring, the dimensional stability is increased to −0.03˜0.08%. Based on these dimensional stability, when the conventional laminate wood flooring having a length of 60 mm is used for an under-floor heating system, cracks of a maximum of 0.48 mm occur. Accordingly, the conventional laminate wood flooring causes consumer complaints. There is, thus, a need to make the size of cracks formed on the laminate wood flooring as small as possible. In addition, since the conventional laminate wood flooring comes off the floor, it has a problem in terms of its practicality.
The dimensional stability after installation is determined in accordance with the following procedure. A wood flooring product is laid on a concrete model having a predetermined size, the product is adhered to the concrete model using an adhesive by a common method, and the resulting structure is left at room temperature for 72 hours to sufficiently cure the adhesive. After the cured structure is heated at 80° C. for 24 hours, the shrinkage of the structure is measured and then expressed in percentage.